Printing

ABSTRACT

An image article comprises a substrate having a security image coated on at least a portion thereof, which security image effects less than 50% reflectance of radiation of a wavelength between 800 and 900 nm, wherein the security image comprises a defined infrared-absorbing compound, for example Pigment Green 8, wherein said infrared-absorbing compound does not create a strongly coloured security image.

FIELD OF THE INVENTION

This invention relates to methods of manufacturing image articles, andimage articles per se.

BACKGROUND TO THE INVENTION

Image articles comprising security images are well known. Securityimages generally comprise an image which is invisible or undetectableunder ambient conditions, and which can be rendered visible ordetectable by application of a suitable stimulus; or alternatively, theimage may change from one colour under ambient conditions to anothercolour upon application of a stimulus.

Security images may be manufactured by coating security inks orcompounds onto a substrate. Examples of known compounds which whencoated onto a substrate provide a security image include photochromiccompounds which generally change from colourless to coloured upon theapplication of ultraviolet light, and thermochromic compounds whichgenerally change from colourless to coloured upon the application ofheat.

Image articles which include security images are useful in many areas ofindustry such as in packaging, identification cards, and labels, forexample. It is useful to provide packaging which includes a securityimage invisible to a user under ambient conditions, but which can berendered visible upon application of a stimulus; for example, if acustoms and excise official wishes to check that imported goods aregenuine goods or whether they are counterfeit goods. If the packagingincludes the security image, rendered visible or detectable by suitablestimulus, the customs and excise official can determine that thepackaging, and hence the goods, are not counterfeit. Likewise, it isadvantageous to provide an identification card in which an image isinvisible or a defined colour under ambient conditions, but which can berendered visible or detectable, or change colour upon application of astimulus in order to prove the identity of a user of the identity card,and in order to determine that the identity card is genuine and not acounterfeit identity card. There are many known examples of suchsecurity images, for example, in the applicant's co-pending applicationsPCT/GB2005/001763 and PCT/GB2005/001766.

In the manufacture of bank notes, it is desirable to include as manysecurity features as possible, which may include multiple securityimages using a variety of compounds capable of changing colour uponapplication of a stimulus (including movement of the bank note to changeviewing angle), or turning coloured from colourless, or vice versa.

In many countries, officials and state authorities use apparatus, suchas third party verifiers, which detect the percentage light absorbanceand/or reflection at a wavelength of approximately 800-900 nm (in theinfrared region), to detect whether specific security images comprisingcompounds which absorb infrared radiation between 800-900 nm arepresent; and hence help to determine whether or not a bank note isgenuine or counterfeit.

It is desirable to provide bank notes which contain security imagescomprising compounds capable of exhibiting 50% or less light reflectanceat approximately 800-900 nm. Many bank notes include Carbon Black as apigment which possesses the characteristic of less than 50% lightreflectance at 800-900 nm. Unfortunately, in order to provide a suitablystrong image, with the required light reflectance characteristics at800-900 nm, Carbon Black is generally needed in a concentration whichproduces a dull grey image in the positions where the Carbon Black islocated, when coated at concentrations generally used (for example, 3%w/w of the total weight of the ink dispersion laid down on the substratepaper for bank notes). Bank note counterfeiters recognise from the dullgrey image that Carbon Black is present in bank notes, and commonly nowuse Carbon Black in order to avoid their counterfeit bank notes beingdetected as counterfeit when utilising third party verifiers to verifythe light reflectance at 800-900 nm.

It would therefore be advantageous to provide a security image on a banknote or any other image article requiring a security image, in which asecurity image includes one or more compounds having a 50% or less lightreflectance at 800-900 nm at a given concentration within an image (andpreferably around 850 nm), and in which the compound utilised does notcreate a strongly coloured image. It would be particularly advantageousto provide such a compound for inclusion in a security image in whichthe compound produces a substantially colourless security image, butwhich has 30% or less light reflectance at 800-900 nm. Most preferably10% light reflectance in the 800-900 region is desired.

It is therefore an aim of the preferred embodiments of the presentinvention to overcome or mitigate at least one problem with the priorart, whether expressly disclosed herein or not.

SUMMARY OF THE INVENTION

In Künstliche Organische Farbstoffe und Ihre Zwischenprodukte, HansRudolf Schweizer (Springer, Verlag 1964, pp 245-246), there is describeda water soluble dye known as Naphthol Green B (C.I. Acid Green 1),useful in promoting the evaporation of water from sea-water to producesea-salt—this works by enhanced heating through NIR absorption fromsolar radiation.

C.I. Acid Green 1 has the structure:

A commercially available pigment analogue of C.I. Acid Green 1 is C.I.Pigment Green 8 which has the structure:

The applicant has surprisingly found that this green infrared-absorbingpigment can be coated in aqueous media, onto a suitable substrate inrelatively low concentrations to produce a security image which exhibitsless than 50% light reflectance at a wavelength of 800-900 nm, whilstexhibiting only a pale green colour on the substrate. The applicantfurther surprisingly found that similar pigments which include differingmetal complexes and salts thereof, and polymers of the same, alsoexhibit light reflectance of less than 50% at a wavelength of 800-900nm, whilst producing very pale, almost colourless images on substratessuch as paper, card, and the like.

It should be noted that the iron (Fe²⁺) complexes of o-nitroso naphtholsor o-nitroso phenols are bright green, along with lanthanide complexes,where as cobalt complexes are brown. The colour thus varies with thechoice of transition element.

Thus, image articles which include security images comprising C.I.Pigment Green 8 and similar compounds, exhibit excellent characteristicsfor security images on bank notes for detection by third party infraredverifiers, whilst being manageable, and able to be coated ontosubstrates from liquid medium.

Accordingly, in a first aspect of the present invention there isprovided an image article comprising a substrate having a security imagecoated on at least a portion thereof, which security image effects lessthan 50% reflectance of radiation of a wavelength between 800 and 900nm, wherein the security image comprises an infrared-absorbing compoundselected from:

or a salt or polymer thereof, wherein

-   -   M is a metal selected from a group 3-10 (Group IIIB-VIII)        element or a lanthanide;    -   R₁ is selected from hydrogen, phosphonate, sulphonate, nitro,        halo, cyano, thiocyano, thioalkyl, thioaryl, alkyl, alkoxy,        aryl, aryloxy, amines, substituted amines and substituted aryl;    -   one of R₂ and R₃ is oxygen and the other of R₂ and R₃ is NO;    -   n is a number corresponding to half the co-ordination number of        the metal M;    -   each L and L′ is independently a ligand complexed to the metal        M; and    -   y is a number corresponding to the co-ordination number of the        metal M;        wherein said infrared-absorbing compound does not create a        strongly coloured security image.

The image article of the present invention comprises a security imagewhich is not strongly coloured. By this it is meant that the imageitself is not strongly coloured: the compound used to form the image mayin fact be strongly coloured when provided in concentrated form, but theamount applied to the substrate results in a security image which is notstrongly coloured.

Suitably the security image is not brightly-coloured.

Preferably it is pale, lightly coloured or colourless. Preferably theinfrared-absorbing compound of the security image is applied atconcentrations such that the image formed has low absorption of light inthe visible range, for example at 400 to 700 nm. Preferably the securityimage formed from the infra-red absorbing compound has a reflectance ofgreater than 50% at wavelengths of 400 to 700 nm.

Because the security image formed from the infra-red absorbing compoundis not strongly coloured, it may be difficult to observe with the nakedeye and/or may be masked by a further image at the same location whichis more strongly coloured. This further image may overlap with some orall of the security image and may or may not be identical with saidsecurity image.

The security image may therefore be regarded as a hidden or covertimage.

According to a second aspect of the present invention there is provideda method of manufacturing an image article comprising the steps of:

(a) providing a substrate; and(b) image-wise coating a compound selected from:

or a salt or polymer thereof, onto at least a portion of the substrateto form a security image which is not strongly coloured and whicheffects less than 50% reflectance of radiation of a wavelength between800 and 900 nm, wherein:

-   -   M, R₁, R₂, R₃, n, L, L′ and y are as described for the first        aspect of the invention.

Suitably, M is selected from iron, cobalt, nickel, aluminium, scandium,chromium, vanadium, titanium, manganese or a lanthanide. Mostpreferably, M is selected from iron, cobalt and lanthanum.

Preferably, M is a metal having a co-ordination number of 6 or 8, and nis correspondingly 3 or 4, and y is correspondingly 6 or 8.

Preferred salts of the compounds coated onto the substrate in the firstand second aspects of the invention include compounds of formula:

wherein M, n and R₁, R₂ and R₃ are as described for the first and secondaspects of the invention, X is a metal cation selected from a group 1 or2 metal (alkali metal and alkaline earth metal) and aluminium, and thesum of m and t correspond to the total number of negative charges on thecompound.

Particularly preferred salts include those of formula:

wherein M, X, n, m and t are as described herein above.

Particularly preferred salts include those having the followingformulae:

Each L and/or L′ is preferably independently selected from chlorine,bromine, hydroxyl, water or pairs of ligands of group L and/or group L′may comprise a single ligand forming a ring structure with metal M, andmay for example be formed from 1,3-dinitroso-2,4-dihydroxybenzene or1,5-dinitroso-2,6-dihydroxy-naphthalene groups connected to the metal Mvia the nitroso and hydroxyl groups.

In particularly preferred embodiments, the compound of the securityimage coated onto the substrate has the formula:

orwherein M, R₂ and R₃ are as described herein above, L1-L6 and L1′-L6′are ligands independently selected from chlorine, bromine, hydroxyl,water, or any number of pairs of L3-L6 and/or L3′-L6′ may be combined ofa single ligand forming a ring structure with metal M, and may forexample be formed from 1,3-dinitroso-2,4-dihydroxybenzene, or1,5-dinitroso-2,6-dihydroxynaphthalene, connected to the metal M via thenitroso and hydroxyl groups.

Polymers of the infrared-absorbing compounds are preferably dendriticpolymers in which each M is complexed to threedinitroso-2,4-dihydroxybenzene or threedinitroso-2,6-dihydroxynaphthalene groups (preferably1,3-dinitroso-2,4-dihydroxybenzene,1,5-dinitroso-2,6-dihydroxynaphthalene or1,5-dihydroxy-4,8-dinitrosonaphthalene).

Particularly preferred polymeric forms of infra-red absorbing compounduseful for the invention are:

and wherein Fe may be replaced with any other metal M as described forthe first or second aspects of the invention.

Suitably, the infrared-absorbing compound is image-wise coated onto thesubstrate in solution, or as a suspension or dispersion of theinfrared-absorbing compound in a suitable medium.

Preferably the infrared-absorbing compound is coated onto the substratein a composition further comprising one or more further pigments and/orone or more dyes. The composition containing the infrared-absorbingcompound and one or more further pigments or dyes may comprise asolution of the one or more further pigments and/or dyes in which isdispersed the infrared-absorbing compound, a suspension or dispersion ofthe one or more further pigments and/or dyes and the infrared-absorbingcompound, or any other suitable form.

Suitably the infrared-absorbing compound is coated onto the substrate asa dispersion or suspension of the infrared-absorbing compound, with orwithout further pigments and/or dyes, in a liquid medium. Intaglio inkbases commonly use tung oil, whilst the offset and letterpress ink basescommonly use linseed oil, an aromatic free mineral oil (boiling range280-310° C.) and/or an aromatic free mineral oil (boiling range 260-290°C.), as liquid carriers. Tolvene, xylene or methylethylketone may alsobe used, for example.

Suitably the composition containing the infrared-absorbing compound iscoated onto the substrate at a concentration of at least 1 gm⁻²(preferably at least 1 μm wet film thickness), more preferably at least2 gm⁻² (preferably at least 2 μm wet film thickness), yet morepreferably at least 4 gm⁻² (preferably at least 4 μm wet filmthickness), and especially at least 6 gm⁻² (preferably at least 6 μm wetfilm thickness). Films ranging from 2 μm to 80 μm wet film thickness areused in the bank note industry. Preferably the infrared-absorbingcompound is coated onto the substrate in an amount capable of absorbinggreater than 50% of infrared radiation impinged or impinging on thesubstrate, more preferably greater than 75%. The use of further pigmentsand/or dyes which in themselves do not absorb and/or reflect infraredradiation at 800-900 nm helps to mask any colour formed by theinfrared-absorbing compound when coated onto a substrate, and thus maskthe presence of the infrared-absorbing compound from would-becounterfeiters.

Suitably the infrared-absorbing compound is coated onto the substrate asa dispersion in a dye- or pigment-containing ink, the ink preferablycontaining at least one oil as the medium in which theinfrared-absorbing compound and/or further pigments or dyes in the inkare dispersed. Suitable oils include natural oils such as linseed oil,and synthetic hydrocarbon or mineral oils.

Suitably the infrared-absorbing compound is coated onto the substrate ina composition at a concentration which results in an image which is notstrongly coloured. In some embodiments, the infrared-absorbing compoundmay be applied at concentrations of less that 4 wt %, for exampleapproximately 1 wt % or 2 wt %.

The infrared-absorbing compound may be included in an ink compositionwhich has a substantially identical colour to a second ink compositionwhich does not contain the infrared-absorbing compound. To prepare sucha combination of ink compositions (as a matched colour pair), theinfrared-absorbing compound is added to a base ink formulation whichresults in a composition of a defined colour. The colour of a secondportion of the base ink formulation is then adjusted (for example by theaddition of known pigments) such that it is visually identical with thedefined colour of the ink composition containing the infrared-absorbingmaterial. Thus when the two compositions are applied to a substrate, theresultant images are visually indistinguishable with the naked eye.

The two ink compositions may be used to form a single image, part of theimage being printed with the ink composition containing theinfrared-absorbing compound and the remainder being printed with itsmatched colour pair ink. When viewed using an infrared camera only thepart of the image containing the infrared-absorbing compound is seen.

Preferable substrates include paper, especially paper used for banknotes such as velin paper, card, metals (including alloy), textiles(including wool, cotton, hemp, jute, linen and flax, as naturaltextiles, and nylon, rayon, polyamide and polyester as synthetictextiles), rubber, ceramics, glass, composite materials, carbon fibre,and any mixture thereof.

Especially preferred substrates are paper and card, and most especiallypaper, such as velin paper, commonly used as bank note substrates.

Preferably the substrate is a sheet substrate and more preferably asubstantially planar sheet substrate. The sheet substrate may be rigidor flexible, but is preferably flexible. The infrared-absorbing compoundmay be image-wise coated on one or both sides of the sheet substrate.

Preferably the image article is a printed article, suitably a paperarticle, which is printed on both sides. Preferably it is printed with acoloured ink on both sides. It may be image-wise coated with theinfrared-absorbing compound to form a security image on one or bothsides.

There may be more than one infrared-absorbing compound coated onto thesheet substrate, and each infrared-absorbing compound may be coatedsimultaneously or sequentially.

The image article may comprise packaging, for example a pharmaceuticalcarton, an article of clothing, a label or the like. It may comprise anidentification document, for example an identification card, a passportor driving licence. The image article may comprise a credit card, avoucher or a ticket, for example a cinema or theatre ticket, or airlineor train ticket.

The image article may be of monetary value. It may for example be ashare certificate, or a stamp certificate or tax voucher (for example avehicle tax disc).

In some embodiments the image article may comprise one or moreadditional security elements. For example it may comprise one or morefurther security images. These security images may comprise compoundsdetectable by infrared radiation or may comprise compounds which aredetectable by other types of radiation, for example ultraviolet orvisible radiation.

Alternatively and/or additionally the one or more additional securityelements may be selected from a hologram, a metallic strip runningthrough the substrate, a watermark or an embossed portion.

The image article may include, as security features, intaglio printing,microprinting, a background image or red or green phosphors.

In especially preferred embodiments, the image article includes, assecurity elements, a mixture of covert and overt features. Overtfeatures are those which may be detected by visible inspection of thearticle, for example the inclusion of a background image. Covertfeatures include those which may be detected in response to a stimulus,for example the application of ultraviolet or infrared light.

Most preferably the image article is a banknote.

In preferred embodiments the security image of the image article of thepresent invention is suitable for inclusion as a security feature on abanknote.

Thus the infrared compound of the security image of the image article ofthe present invention typically exhibits good light-fastness. Preferablyit exhibits good wash fastness. Preferably it exhibits good solventfastness. Preferably the infrared compound of the security image of thepresent invention exhibits sufficient light-fastness and wash-fastnessfor it to be suitable for inclusion in a banknote.

There are typically 22 fastness tests that a banknote may be subjectedto in order to determine the suitability of any security featurespresent. These include chemical fastness to the following solvents:xylene, hydrochloric acid, sodium hydroxide solution,tetrachloroethylene.

To assess the suitability of an image article of the present inventionfor use as a banknote, the infrared absorbance of the security image maybe measured and then the image article is immersed at room temperaturein a beaker of the appropriate solvent for 30 minutes. The image articleis removed, dried and its infrared absorbance re-measured. Any change inabsorbance is then rated on a scale of 0-4, 4 being no change and 0representing a substantial (greater than 50%) change.

The image article may also be subjected to conventional launderingwash-fastness tests and also light-fastness tests. For the wash-fastnesstest, the infrared absorbance of the security image of the image articleis measured and then it undergoes a domestic home washing test cycle inwhich it is washed in a suitable detergent solution. After washing, theinfrared absorbance of the security image of the image article isre-measured and any change in absorbance is again rated on a suitablescale.

The light-fastness test involves subjecting the image article toaccelerated light fading in a xenon light chamber. The infraredabsorbance of the security image of the image article is measured and itis then placed in a xenon light chamber along with a series of 8 bluewool standards and exposed to xenon light. The infrared absorbance ofthe security image is determined as each blue wool standard fades. Thelight-fastness is rated as the highest blue wool standard at which nosignificant change in infrared absorbance is noted. Blue wool standard 8represents the highest level of light-fastness, whilst 1 represents thelowest level. A security image suitable for use on a bank note has ablue wool standard of at least 4.

Preferably the security image of the image article of the composition ofthe present invention exhibits a light-fastness which is equivalent toat least blue wool standard 5, more preferably at least blue woolstandard 6.

According to a third aspect of the present invention there is provideduse of a compound of formula:

or a salt or polymer thereof, coated on at least a portion of thesubstrate, as an infrared radiation-absorbing additive in a securityimage coated on a substrate of an image article, wherein:

-   -   M is a metal selected from a group 3-10 (Group IIIB-VIII)        element or a lanthanide;    -   R₁ is selected from hydrogen, phosphonate, sulphonate, nitro,        halo, cyano, thiocyano, thioalkyl, thioaryl, alkyl, alkoxy,        aryl, aryloxy, amines, substituted amines and substituted aryl;    -   one of R₂ and R₃ is oxygen and the other of R₂ and R₃ is NO;    -   n is a number corresponding to half the co-ordination number of        the metal M;    -   each L and L′ is independently a ligand complexed to the metal        M; and        y is a number corresponding to the co-ordination number of the        metal M;        wherein said security image is not strongly coloured.

Suitably the infrared absorbing compound, coating, substrate and imagearticle are as described for the first aspect of the invention.

According to a fourth aspect of the present invention there is provideda method of verifying the authenticity of an image article of the firstaspect, the method comprising exposing said image article to radiationhaving a wavelength of between 800 and 900 nm, and measuring thereflectance of said radiation.

For an authentic image article, the reflectance in the region of thesecurity image is suitably less than 50%.

The method of the fourth aspect may be carried out using any suitabledetector. One suitable device is a Shimadzu UV-3101 PC UV-VIS-NIRscanning spectrophotometer. An infrared camera could also be used.

Typically radiation is applied and the reflectance thereof is measured,thus allowing absorbance to be calculated. Preferably the method of thefourth aspect employs a reader device. The reader device may comprise aninfrared emitter and an infrared detector.

The method may further comprise measuring the extent of the absorptionof infrared radiation at a selected wavelength. Thus the percentageabsorbance or reflectance can be measured.

The method of the fourth aspect of the present invention may in someembodiments permit a quick, non-quantitative determination of thepresence or otherwise of an infrared-absorbing material, by quicklychecking for broadband adsorption or absorption at specific wavelength.

Alternatively, the method may be used to measure quantitatively theextent of absorbance at a specific wavelength. The more accurately theinfrared absorption spectrum of an article can be measured, the moredifficult it would be to counterfeit such an article.

The reader device could be built into a machine, for example a passportscanner, a chip-and-pin device, or an ATM. Alternatively a reader devicecould be supplied independently as a mobile device. The reader devicemay for example be provided as a hand held pen type detection devicewhich would offer a pass/fail response on scanning a sample.

The method of the fourth aspect may be carried out periodically onrandomly selected articles or it may be carried out routinely on everyarticle. For example, a photosensitive diode could be included in a cashmachine to measure the IR absorbance at a given wavelength of eachbanknote. Thus, a counterfeit banknote could be easily detected.

EXAMPLES

For better understanding of the various aspects of the invention and toshow how embodiments of the same may be put into effect, the inventionwill now be described by way of the following non-limiting examples.

Example 1

A green infrared-absorbing pigment (Pigment Green B, CI Pigment Green 8)was synthesised in accordance with the following procedure; thestructure of the pigment being given below in

2-Naphthol (10 g, 0.07M) was dissolved in a warm solution of sodiumhydroxide (2.8 g, 0.07M) in distilled water (120 ml). The solution wascooled to 0-5° C. and sodium nitrite (5 g, 0.073M) was added. Thesolution was stirred and 5.6M sulphuric acid (17 ml) was slowly addedover 90 minutes; the solution being kept at 0-5° C. throughout theaddition of the acid. The solution was stirred for a further 1 hourafter the addition of the acid was complete, after which time sodiummetabisulphite (13.3 g, 0.07M) was added and the suspension stirreduntil the nitroso compound had completely dissolved to yield a greensolution; the solution being stirred for a further 30 minutes afterwhich time the pH of the solution was adjusted to 6.5. A solution ofiron II sulphate heptahydrate (6.4 g, 0.023M) in distilled water (10 ml)was added, followed by a small amount of sodium hydroxide solution toyield a green precipitate. The precipitate was stirred for 30 minutesand then collected by filtration. The precipitate was washed thoroughlywith distilled water and finally dried in a vacuum desiccator. The ironII complex was purified by dissolving it in a minimum amount ofdimethylformamide, filtering the solution to remove any solid andfinally precipitating the iron II complex via the addition of water. Thegreen iron II complex (Pigment Green B) was collected by filtration anddried in a vacuum desiccator.

Example 2

The process described in Example 1 was repeated, but in this case cobaltII chloride hexahydrate (5.5 g, 0.023M) was added in place of the ironII sulphate heptahydrate to yield the brown cobalt complex.

Example 3

The process described in Example 1 was repeated, but in this caselanthanum III chloride hexahydrate (8.6 g, 0.023M) was added in place ofthe iron II sulphate heptahydrate to yield the light green lanthanumcomplex.

Example 4

The process described in Example 1 was repeated, but in this casealuminium potassium sulphate dodecahydrate (10.9 g, 0.023M) was added inplace of the iron II sulphate heptahydrate to yield the green aluminiumcomplex.

Example 5

The process described in Example 1 was repeated, but in this caseresorcinol (7.7 g, 0.07M) was used in place of 2-naphthol to yield adendritic polymeric green iron complex; the pigment being fast towashing and light and sold commercially as Solid Green 0. The structureof the 3:1 polymeric iron complex is given below.

Example 6

A further polymeric green infrared-absorbing pigment was synthesised viathe nitrosation of 2,6-dihydroxynaphthalene with nitrous acid and itssubsequent complexation with iron. Thus, 2,6-dihydroxynaphthalene (2.8g, 0.018M) was dissolved in a warm solution of sodium hydroxide (5.6 g,0.036M) in distilled water (40 ml). The solution was cooled to 0-5° C.and sodium nitrite (2.5 g, 0.036M) was added. The solution was stirredand 5.6M sulphuric acid (9 ml) was slowly added over 90 minutes; thesolution being kept at 0-5° C. throughout the addition of the acid. Thesolution was stirred for a further 1 hour after the addition of the acidwas complete, after which time sodium metabisulphite (6.84 g, 0.036M)was added and the suspension stirred until the nitroso compound hadcompletely dissolved to yield a green solution; the solution beingstirred for a further 30 minutes after which time the pH of the solutionwas adjusted to 6.5. A solution of iron II sulphate heptahydrate (1.67g, 0.006M) in distilled water (10 ml) was added, followed by a smallamount of sodium hydroxide solution to yield a green precipitate. Theprecipitate was stirred for 30 minutes and then collected by filtration.The precipitate was washed thoroughly with distilled water and finallydried in a vacuum desiccator. The polymeric iron II complex was purifiedby dissolving it in a minimum amount of dimethylformamide, filtering thesolution to remove any solid and finally precipitating the iron IIcomplex via the addition of water. The iron II complex was collected byfiltration and dried in a vacuum desiccator. The structure of the greenpolymeric iron II complex formed is given below:

Example 7

Naphthol Green B (CI Acid Green 1) is a water-soluble sulphonatedderivative of Pigment Green B which can be readily converted to awater-insoluble pigment via conversion to its strontium, calcium,barium, magnesium, aluminium or zinc salt. The structure of NaphtholGreen B is given below:

Alternatively some authors (Zollinger, Color Chemistry, Syntheses,Properties, and Applications of Organic Dyes and Pigments, 3^(rd)Edition, Wiley-VCH, 2003) write the structure of this dye as:

Thus, Naphthol Green B (4.4 g) was dissolved in distilled water (75 ml)and heated at 35° C. An alkaline solution of rosin, prepared bydissolving rosin (0.83 g) and sodium hydroxide (0.3 g) in distilledwater (30 ml) at 50° C., and an aqueous solution of barium chloride,prepared by dissolving barium chloride dihydrate (4.0 g) in distilledwater (30 ml), were simultaneously added to the stirred dye solution at35° C. The dye solution was raised to the boil and boiled for 10minutes, after which time cold distilled water (100 ml) was added tocool the pigment suspension. The precipitated green barium pigment wascollected by filtration, washed thoroughly with cold distilled water andthen dried in a vacuum desiccator overnight.

Example 8

The process described in Example 7 was repeated, but in this casecalcium chloride hexahydrate (4.0 g) was used in place of bariumchloride dihydrate to precipitate the dye as a water-insoluble greenpigment.

Example 9

The process described in Example 7 was repeated, but in this casestrontium chloride hexahydrate (4.0 g) was used in place of bariumchloride dihydrate to precipitate the dye as a water-insoluble greenpigment.

Example 10

The process described in Example 7 was repeated, but in this casemagnesium chloride hexahydrate (4.0 g) was used in place of bariumchloride dihydrate to precipitate the dye as a water-insoluble greenpigment.

Example 11

The process described in Example 7 was repeated, but in this case zincacetate dihydrate (4.0 g) was used in place of barium chloride dihydrateto precipitate the dye as a water-insoluble green pigment.

Example 12

The process described in Example 7 was repeated, but in this casealuminium potassium sulphate dodecahydrate (4.0 g) was used in place ofbarium chloride dihydrate to precipitate the dye as a water-insolublegreen pigment.

Example 13

An intaglio security ink was prepared by dispersing aninfrared-absorbing pigment in a commercially available intaglio inkformulation. Thus, the infrared-absorbing pigment (0.5 g) synthesised inExample 1 was dispersed in a yellow intaglio ink base (24.5 g)(Gleitsmann Security Inks GmbH) on a triple roll mill; the intaglio inkbase being formulated as follows in Table 1:

TABLE 1 Intaglio ink base formulation Component Weight (%) Modifiedvehicle* 38.0 Pigment 2.0 Calcium carbonate 49.6 Polyethylene wax 8.0(micronised) Drier (10% manganese 0.3 octoate) Drier (18% cobalt 0.1octoate) Aliphatic mineral oil 2.0 (boiling range 170-260° C.) *Themodified vehicle was composed of a commercial vehicle/varnish (80%),Trionol HK 9 (Lawter International, Belgium) and bodied tung oil (20%).

Proof prints of the intaglio infrared-absorbing security ink wereprepared on velin paper using a Prüfbau proof printer; the inks beingprinted at a film thickness of 90.0 gm⁻². The IR absorbance of theresulting print was measured on a Shimadzu UV-3101 UV-VIS-NIRspectrophotometer incorporating a reflectance head attachment; the printexhibiting an IR absorbance of 94.2% at a wavelength of 800 nm.

Example 14

A letter-press security ink was prepared by dispersing aninfrared-absorbing pigment in a commercially available letter-press inkformulation. Thus, the infrared-absorbing pigment (0.5 g) synthesised inExample 1 was dispersed in a yellow letter-press ink base (24.5 g)(Gleitsmann Security Inks GmbH) on a triple roll mill; the letter-pressink base being formulated as follows in Table 2:

TABLE 2 Intaglio ink base formulation Component Weight (%) Varnish* 63.5Pigment 4.5 Calcium carbonate 22.3 Linseed oil 5.1 Aliphatic mineral oil4.0 (boiling range 260-310° C.) Hydroquinone 0.3 Drier (10% manganese0.2 octoate) Drier (18% cobalt 0.1 octoate) *The varnish was composed ofa modified phenolic resin (40%), linseed oil (20%), aromatic freemineral oil (boiling range 280-310° C.) (20%), aromatic free mineral oil(boiling range 260-290° C.) (19.3%) and aluminium (ethylacetoacetonato)isopropoxide (0.7%).

Proof prints of the letter-press infrared-absorbing security ink wereprepared on velin paper using a Prüfbau proof printer; the inks beingprinted at a film thickness of 4.0 gm⁻². The IR absorbance of theresulting print was measured on a Shimadzu UV-3101 UV-VIS-NIRspectrophotometer incorporating a reflectance head attachment; the printexhibiting an IR absorbance of 61.3% at a wavelength of 800 nm.

Example 15

An offset security ink was prepared by dispersing an infrared-absorbingpigment in a commercially available offset ink formulation. Thus, theinfrared-absorbing pigment (1.0 g) synthesised in Example 1 wasdispersed in a yellow offset ink base (24.0 g) (Gleitsmann Security InksGmbH) on a triple roll mill; the offset ink base being formulated asfollows in Table 3:

TABLE 3 Intaglio ink base formulation Component Weight (%) Varnish* 67.0Pigment 4.5 Calcium carbonate 9.2 Linseed oil 13.7 Polyethylene wax 5.0Hydroquinone 0.3 Drier (10% manganese 0.2 octoate) Drier (18% cobalt 0.1octoate) *The varnish was composed of a modified phenolic resin (40%),linseed oil (20%), aromatic free mineral oil (boiling range 280-310° C.)(20%), aromatic free mineral oil (boiling range 260-290° C.) (19.3%) andaluminium (ethylacetoacetonato) isopropoxide (0.7%).

Proof prints of the offset infrared-absorbing security ink were preparedon velin paper using a Prüfbau proof printer; the inks being printed ata film thickness of 2.0 gm⁻². The IR absorbance of the resulting printwas measured on a Shimadzu UV-3101 UV-VIS-NIR spectrophotometerincorporating a reflectance head attachment; the print exhibiting an IRabsorbance of 62.5% at a wavelength of 800 nm.

Example 16

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 2 (2.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 46.2%at a wavelength of 800 nm.

Example 17

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 3 (1.25 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 19.2%at a wavelength of 800 nm.

Example 18

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 4 (1.25 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 35.7%at a wavelength of 800 nm.

Example 19

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 5 (0.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 2.0 gm⁻²; the prints exhibiting an IR absorbance of 59.8%at a wavelength of 800 nm.

Example 20

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 6 (2.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 47.8%at a wavelength of 800 nm.

Example 21

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 7 (2.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 92.4%at a wavelength of 800 nm.

Example 22

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 8 (2.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 63.3%at a wavelength of 800 nm.

Example 23

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 9 (2.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 61.3%at a wavelength of 800 nm.

Example 24

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 10 (20.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 77.3%at a wavelength of 800 nm.

Example 25

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 11 (2.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 63.5%at a wavelength of 800 nm.

Example 26

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 12 (k.5 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 59.7%at a wavelength of 800 nm.

Example 27

The process described in Example 1 was repeated, but in this case1-naphthol was used instead of 2-naphthol a water-insoluble greenpigment; the structure of the pigment being given below:

Example 28

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 26 (1 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 2.0 gm²; the prints exhibiting an IR absorbance of 61.5% ata wavelength of 800 nm.

Example 29

A 3:1 iron complex of 2,3-dihydroxynaphthalene was synthesised via thefollowing process. 2,3-Dihydroxynaphthalene (0.0051M, 0.82 g) (Aldrich)was dissolved in methanol (50 ml) and gradually added to a solution ofiron II sulphate heptahydrate (0.0017M, 0.5 g) (Aldrich) in distilledwater (50 ml). The solution was heated to the boil for 5 minutes toyield a navy blue/black pigment. The pigment was collected byfiltration, washed thoroughly with cold water and dried in a vacuumdesiccator. The structure of the pigment is given below:

Structure of the 3:1 Iron Complex of 2,3-dihydroxynaphthalene Example 30

The process described in Example 15 was repeated, but in this case theIR absorber synthesised in Example 29 (1 g) was used instead of thatsynthesised in Example 1. Prüfbau proof prints were prepared at a filmthickness of 6.0 gm⁻²; the prints exhibiting an IR absorbance of 59.2%at a wavelength of 800 nm.

Example 31

Colour pairs were prepared with the following ink compositions byshading the base ink colour to match the colour of the ink obtainedafter the addition of Pigment Green B.

Pigment Green B IR (%) added reflectance to base (%) between Base inkImage colour ink 800-840 nm IR Transparent Pale green 5 40 white IRTransparent Light green 10 28 white IR Transparent Green 15 23 whiteBase Ink 680141 Greenish-blue 12 27 Base Ink 680150 Bluish-green 12 20Base Ink 680102 Yellowish- 12 25 green Base Ink 680101 Green 12 27

Example 32 (Comparative)

Aminium dyes: Water-soluble cationic dyes were converted towater-insoluble pigments by complexation with phosphotungstic acid. Theparent dyes, A191, A192 and A207 were supplied by Gentex Optics Inc(USA). However, their maximum IR absorbance occurred between 900-1200 nmand so was outside the desired absorption band of 800-900 nm. Thepigments are darkly coloured blues and greens, and produce stronglycoloured prints even at low concentrations. They also failed to meet thechemical resistance requirements, the complexes being destroyed insodium hydroxide solution.

Example 33 (Comparative)

Polycylic vat dyes: Water-insoluble dyes such as Cibanon Green BF-MD, CIVat Green 1 (Ciba) and Cibanon Blue BOA-01, CI Vat Blue 20, (Ciba)exhibited suitable light-fastness and chemical resistance fastnessproperties, but failed to achieve acceptable IR absorbance over the800-900 nm band required. They are also highly coloured, and asrelatively high concentrations must be used to achieve the desired IRabsorption characteristics, strongly coloured image areas are produced.

Example 34 (Comparative)

Cyanine dyes: Water-soluble soluble cationic dyes were converted towater-insoluble pigments by complexation with phosphotungstic acid. Theparent dyes, IR-792 perchlorate anddimethyl{4-[1,5,5-tris(4-dimethylaminophenyl)-2,4-pentadienylidene]-2,5-cyclohexadien-1-ylidene}ammonium perchlorate were supplied by Aldrich (UK). These dyes offeredsuitable characteristics, but suffered from poor light-fastnessproperties, Blue Wool Standard 3 compared with a Blue Wool Standard ofat least 6 for Pigment Green B.

Example 35 (Comparative)

Squarilium dyes: A colourless water-soluble anionic dye was converted toa water-insoluble pigment by forming its calcium, barium or strontiumlakes. The dye used was a research sample synthesised in the Departmentof Colour and Polymer Chemistry, University of Leeds. This dye waschosen because of its colourless nature and strong IR absorptioncharacteristics. However, the water-insoluble lakes of this particularsquarilium dye failed to offer suitable IR characteristics over thedesired 800-900 nm range.

1. An image article comprising a substrate having a security imagecoated on at least a portion thereof, which security image effects lessthan 50% reflectance of radiation of a wavelength between 800 and 900nm, wherein the security image comprises an infrared-absorbing compoundselected from:

or a salt or polymer thereof, wherein M is a metal selected from a group3-10 (Group IIIB-VIII) element or a lanthanide; R₁ is selected fromhydrogen, phosphonate, sulphonate, nitro, halo, cyano, thiocyano,thioalkyl, thioaryl, alkyl, alkoxy, aryl, aryloxy, amines, substitutedamines and substituted aryl; one of R₂ and R₃ is oxygen and the other ofR₂ and R₃ is NO; n is a number corresponding to half the co-ordinationnumber of the metal M; each L and L′ is independently a ligand complexedto the metal M; and y is a number corresponding to the co-ordinationnumber of the metal M; wherein said infrared-absorbing compound does notcreate a strongly coloured security image.
 2. An image article accordingto claim 1 wherein M is selected from iron, cobalt and lanthanum.
 3. Animage article according to claim 1 wherein the infrared absorbingcompound is a salt of formula:

wherein M, n and R₁, R₂ and R₃ are as defined in claim 1, X is a metalcation selected from a group 1 or 2 metal and aluminium, and the productof m and t corresponds to the total number of negative charges on thecompound.
 4. An image article according to claim 3 wherein the salt isselected from:


5. An image article according to claim 1 wherein the infrared absorbingcompound is of the formula:

wherein M, R₂ and R₃ are as defined in claim 1, L1-L6 and L1′-L6′ areligands independently selected from chlorine, bromine, hydroxyl, water,or any number of pairs of L3-L6 and/or L3′-L6′ may be combined as asingle ligand forming a ring structure with metal M, and may be formedfrom 1,3-dinitroso-2,4-dihydroxybenzene, or1,5-dinitroso-2,6-dihydroxynaphthalene, connected to the metal M via thenitroso and hydroxyl groups.
 6. An image article according to claim 1wherein the infrared-absorbing compound is a dendritic polymer in whicheach M is complexed to three dinitroso-2,4-dihydroxybenzene or threedinitroso-2,6-dihydroxynaphthalene groups (preferably1,3-dinitroso-2,4-dihydroxybenzene,1,5-dinitroso-2,6-dihydroxynaphthalene or1,5-dihydroxy-4,8-dinitrosonaphthalene).
 7. An image article accordingto claim 6 in which the infra-red absorbing compound is selected from:


8. An image article according to claim 1 wherein the article is abanknote.
 9. An image article according to claim 1 further comprises oneor more additional security elements.
 10. An image article according toclaim 9 which includes covert and overt security elements.
 11. (A methodof manufacturing an image article comprising the steps of: (a) providinga substrate; and (b) image-wise coating an infra-red absorbing compoundselected from:

or a salt or polymer thereof, onto at least a portion of the substrateto form a security image which is not strongly coloured and whicheffects less than 50% reflectance of radiation of a wavelength between800 and 900 nm, wherein: M is a metal selected from a group 3-10 (GroupIIIB-VIII) element or a lanthanide; R₁ is selected from hydrogen,phosphonate, sulphonate, nitro, halo, cyano, thiocyano, thioalkyl,thioaryl, alkyl, alkoxy, aryl, aryloxy, amines, substituted amines andsubstituted aryl; one of R₂ and R₃ is oxygen and the other of R₂ and R₃is NO; n is a number corresponding to half the co-ordination number ofthe metal M; each L and L′ is independently a ligand complexed to themetal M; and y is a number corresponding to the co-ordination number ofthe metal M.
 12. A method according to claim 10 wherein in step (b) thecompound is image-wise coated onto the substrate in a compositioncontaining the infrared absorbing compound and one or more furtherpigments and/or one or more dyes.
 13. The use of a compound of formula:

or a salt or polymer thereof, coated on at least a portion of thesubstrate, as an infrared radiation-absorbing additive in a securityimage coated on a substrate of an image article, wherein: M is a metalselected from a group 3-10 (Group IIIB-VIII) element or a lanthanide; R₁is selected from hydrogen, phosphonate, sulphonate, nitro, halo, cyano,thiocyano, thioalkyl, thioaryl, alkyl, alkoxy, aryl, aryloxy, amines,substituted amines and substituted aryl; one of R₂ and R₃ is oxygen andthe other of R₂ and R₃ is NO; n is a number corresponding to half theco-ordination number of the metal M; each L and L′ is independently aligand complexed to the metal M; and y is a number corresponding to theco-ordination number of the metal M; wherein said security image is notstrongly coloured.
 14. A method of verifying the authenticity of animage article as defined in claim 1, the method comprising exposing saidimage article to radiation having a wavelength of between 800 and 900nm, and measuring the reflectance of said radiation.